UIL Holdings, the parent company of United Illuminating, said it acquired a liquefied natural gas storage facility and related assets in Milford from subsidiaries of Iberdrola for about $20 million.

UIL purchased the assets through its subsidiary on July 31. The Milford facility is operated by Southern Connecticut Gas, which UIL acquired from Iberdrola in 2010 as part of a larger deal that also included Connecticut Natural Gas and Berkshire Gas.

"Our natural gas supply system is designed to provide gas to firm customers under all weather conditions, and it works," James P. Torgerson, president and CEO of UIL Holdings, said in a statement. "This transaction is another prudent investment we are making to ensure we meet customers' demands now but is inclusive of our growing customer base."

Including the Milford facility, which has a storage capacity of 14.6 million gallons, there are three liquefied natural gas plants, with the other two facilities in Rocky Hill and Whately, Mass. The latter two facilities have a combined storage capacity of more than 14.6 million gallons.

Liquefied natural gas traditionally is used to supplement natural gas supplies, particularly during the peak periods of winter. When it is needed, the liquefied gas is heated, converted to vapor, and injected into distribution pipelines.

Shares of UIL Holdings, traded on the New York Stock Exchange under the symbol "UIL," fell 27 cents, or 0.8 percent, to $34.85 on Tuesday.

The company serves more than 700,000 electric and natural gas utility customers in 66 communities across two states. It employs more than 1,850 people in the New England region.

The New Haven-based company is still trying to buy Philadelphia Gas Works, the nation's largest municipally-owned gas utility that agreed to $1.86 billion deal in March. The Philadelphia City Council failed to schedule hearings on the proposed sale before breaking for summer recess last month.

"We have decided to continue our efforts to pursue this acquisition and become a new business partner in the City of Philadelphia," Torgerson said in a July statement.